The campaign has been bitter, running for several weeks, with personal and policy attacks from all sides. Mr Coyne has criticised Mr McCluskey’s strong support for Labour leader Jeremy Corbyn, accusing the union of becoming too political under his leadership.

Mr McCluskey has hit back, saying it made sense for Labour’s biggest union affiliate to be close to the party.

Mr Coyne was criticised by Mr McCluskey for being too close to a number of Labour MPs in the West Midlands who opposed Mr Corbyn’s leadership.

Although he has been suspended from his post as regional secretary, he remains a Unite member, so his supporters say he could still become general secretary if he wins.

It is believed that the ballot papers were being verified on Thursday but actual counting will not start until Friday.

The email sent to Unite staff and members in the West Midlands said: “This is to advise that Gerard Coyne has been suspended from his duties as regional secretary pending an investigation into certain issues which have arisen.

“The investigation is, of course, being conducted under procedure, and it should not be assumed that any offence has necessarily been committed.

“You will appreciate that it is not possible to comment further on this matter.”